1. Field of the Invention
The present invention relates to an image forming apparatus and an image density adjusting method.
2. Description of the Related Art
A typical color image forming apparatus, such as a tandem-type electrophotographic image forming apparatus, has both a monochrome mode and a color mode. Japanese Patent Application Laid-open No. H7-66953 discloses a color image forming apparatus in which only a black and white (B&W) image is formed in the monochrome mode to prolong the life of image carriers, and a full color image is formed in the color mode by superimposing a plurality of color images.
Such an image forming apparatus having the monochrome mode and the color mode generally includes an image quality adjusting unit that forms a pattern of image patches (adjustment patches), reads the density of the pattern using a sensor, and performs a feedback control based on the density of the pattern read by the sensor to optimize the image quality. In a conventional image forming apparatus that includes a plurality of image forming units corresponding to, for example, yellow (Y), magenta (M), cyan (C), and black (Bk), the density of all the four colors, including black, is adjusted at a time.
A color image is not formed in the monochrome mode. Therefore, if the color adjustment is necessary and if the current mode is the monochrome mode, the printing mode must be switched to the color mode. The time for switching the printing mode causes a waiting time to a user, which can be annoying.
Some image forming apparatuses are configured not to perform the color adjustment in the monochrome mode even when the color adjustment is required. In such image forming apparatuses, however, sudden switching of the printing mode from the monochrome mode to the color mode accompanies a color adjustment, and the user has to wait until the color adjustment is completed.
In the image forming apparatus disclosed in Japanese Patent Application Laid-open No. H7-66953, a correction pattern is formed on a transfer medium and a color misalignment is corrected using the correction pattern. However, it cannot be said that the color adjustment is performed considering the problem of switching the printing mode between the monochrome mode and the color mode.